The present invention relates to a method and an apparatus for producing displays of several related but independently variable parameters of an occurrence.
The invention is particularly applicable to the evaluation of test data produced by an eddy current detector and which contains information relative to the position of the detector relative to the body of the detector, the amplitude of the detector output signal, and phase shifts between that output signal and a phase reference. Such testing is performed, for example, to detect flaws, such as cracks, in steam generator tubes.
Known eddy current detectors include two coils mounted in adjacent arms of a bridge. An alternating current is passed through the bridge to generate an electromagnetic field. This field will be influenced by an adjacent metal body to vary the impedance of one or both coils, thereby creating an unbalance voltage in the bridge, which voltage is the difference between the voltages across the two coils. Thus, the waveform of the unbalance voltage is constituted by a carrier wave at the alternating current frequency modulated by the difference between the impedance variations in the two coils.
FIG. 1 illustrates the voltages induced across the two coils of an eddy current detector when moving past a flaw in the body being tested. The respective voltage curves 25 and 26 are offset in time because they are spaced apart in the direction of movement of the eddy current detector. The curves shown represent the modulation of the carrier wave, or waves, which are not represented in FIG. 1. Therefore, the phase variations ocurring in the carrier wave, or waves, are not represented.
The duration of each curve 25, 26 depends on the length of the flaw in the direction of detector movement while the peak amplitude of each curve depends on the extent of the flaw. The carrier wave shift in the coil voltage is a function of the nature, and particularly the depth, of the faw.
FIG. 2 shows curve 28 representing the resulting bridge unbalance voltage which is the difference between curves 25 and 26 of FIG. 1.
Heretofore, evaluation of eddy current test data has involved generation of a strip chart constituted by the waveform of the bridge unbalance voltage amplitude, as the ordinate, with respect to displacement of the eddy current detector along a surface of the metal body being tested, as the abscissa.
Such evaluation further includes analysis of the phase displacement between the applied alternating current and the unbalance bridge voltage for a section of the strip chart in which the waveform has an unusual configuration. This can be achieved by obtaining representations of the unbalance voltage components which are in phase with and in quadrature, or 90.degree. out of phase, with the alternating current applied to the bridge.
Then a Lissajous figure representing a plot of the in-phase amplitude vs. the quadrature phase amplitude can be generated to permit more detailed analysis of the defect which produced the associated waveform segment. For example, a Lissajous figure permits determination of the depth of a crack or of the type of defect. However, a Lissajoua figure does not contain any information relating to detector position along the test body surface. This general technique is described, for example, in U.S. Pat. No. 3,895,290.
Thus, the above-described procedure requires two independent displays, neither of which contains all available information relating to detector position, unbalance voltage amplitude and unbalance voltage phase.
It is also known to generate two strip charts each depicting the amplitude of a respective unbalance voltage component relative to detector position. In this case, phase information is still not present in a readily observable form.